Electrical contact



Dec. 1l, 1945. E` ARSEN ETAL 2,390,595

ELECTRICAL CONTACT Filed Feb. 5, 1945 Earl I Zarsen 76741014 E'. Heil dlji" mazy IN VE NTORS BY @MW @mm HTTORNEY Ratented Dec. l1, 1945ELECTRICAL contraer Earl I. Larsen, Vernon E. Heil, vand Earl F. Swazy,Indianapolis, Ind., assignors to P. B. Mallory & Co., Ine.,Indianapolis, Ind., a corporation of- Delaware Application February 5,1943, Serial No. 474,852

4 claims. (ol. zoo-166) This invention relates to electric contacts andmethod of making the same.

An object of the invention is to improve electric contacts.

Other objects of the invention will be apparent from the description andclaims.

The drawing shows an electric contact embodying features of theinvention.

Many electric contacts are produced from metal compositions of a powdercomponent such as a refractory metal or refractory metal compound orothermaterial of a non-fusing nature interspersed with a lower meltingpoint high conductivity metal such as silver or copper or their alloys.For many operations such contacts of the prior art are very satisfactoryand give excellent performance. However, in some applications,especially where high current circuits are made and broken by thecontacts or the circuits are subject to high overload currents ortransients, it has been found that sticking together or welding of suchcontacts often occurs during the rst few operations.

We have discovered that such contacts can be improved and the occurrenceof sticking or welding practically eliminated by etching the surface ofsuch contacts before they are put into operation. It is believed thatthe sticking or welding takes place as a result of the presence of afilm of the high conductivity metal such as silver or copper on thecontact face. These metals and their alloys are of comparatively lowmelting point and if they form the actual contacting layers a highcurrent surge may cause momentary fusion of these metals on the contactsurfaces resulting in welding the contacts together.

The process of the present invention comprises etching the contactsurfaces in a suitable acid or other etching bath which will selectivelydissolve or etch out the high conductivity metal such as silver orcopper to a depth sufilcient to expose the more refractory contactconstituent such as the refractory metal or refractory metal compound orother less fusible constituents which may be present.

The invention has application to contacts of various compositions suchas those wherein the more refractory constituents are tungsten,molybdenum, tungsten carbide, molybdenum carbide and the refractorycarbides. borides and ntrides of the metals of the 4th, 5th and 6thgroups of the periodic table combined with silver, copper, silvercopperalloys, silver base alloys and copper base alloys. The invention is alsoapplicable to electric contacts containing cadmium oxide as the morerefractory constituent such as the contacts described in Hensel Patent2,145,690. One such composition comprises cadmium oxide 10%, silver Theinvention is also applicable to an electric contact containing an irongroup metal such Electric contacts of the type described areYmanufactured by various processes such as by mixing all the constituentstogether as powders and pressing them or mixing and pressing them andsubsequently sintering them. They can also be produced, especially wherehigher proportions of the refractory constituents are used, by pressingthe refractory powders together and sintering them and subsequentlylmpregnating the sintered compact with the lower melting point highconductivity metal. The contacts may also be fabricated by mixing thepowders, pressing and heating to a suillciently high temperature tocause impregnation of all or a portion of the lower melting point highconductivity material and coincidental sintering of the refractorymetal. In any event, it will be apparent that some of the lower meltingpoint metal will appear on the contact surface.

In preparing such contacts for use it is sometimes necessary to performmachining operations on them and in this case it is believed that someof the lower melting point metal such as silver becomes smeared over thecontact surface. Also during the machining operation the refractorycomponent may be pulled out of the contact surface leaving the lowermelting point metal exposed.

In practising the present invention, the etching is performed by placingthe contacts in a suitable etching bath and allowing them to remain fora period of time sufficient to dissolve the high conductivity metal fromthe contact surface and expose the more refractory particles.

One suitable etching bath for dissolving silver is produced by adding 30cc. of a saturated solution of potassium dichromate to cc. of 5%sulfuric acid. The contacts are placed in this solution and are allowedto remain therein for approximately one minute at room temperature.

Another etching solution that is satisfactory is one composed of 50%nitric acid and 50% water. Concentrated nitric acid may be used althoughit attacks the silver very rapidly and is dicult to control.

The above solutions are suitable for etching any of the describedcontact compositions containing silver.

Contacts containing copper as the high conductivty metal may be etchedin a solution of of 30% hydrogen peroxide solution and 8% ammoniumhydroxide.

In one instance contacts formed of 10% cadmium oxide and 90% silver wereused on a heavy duty relay. Prior to applicants invention, there wereoccurrences where the contacts stuck together the first time they-wereoperated. On relays fltted with contacts of the same composition whichhad been etched as described herein, no sticking was encountered for thesame service. Further tests were made under even more rigorousconditions, including operations of the contacts for controllingcurrents of 2,000 amperes without sticking. Extended-life tests weremade on the contacts operating at 200 amperes at the rate of oneoperation per second without failure in a test extending to 50,000operations. Moreover, during this test the contact resistance was foundto 'be' appreciably less than 100 millivolts at 200 amperes throughoutthe entire run.

Similar results were obtained with contacts formed of 50% tungstencarbide and 50% silver.

Referring to the drawing, the contact body I comprises a disc formed ofa mixture of refractory constituent and a lower melting pointconstituent. The contact disc I is secured to a contact support arm 2 bya layer 3 of brazing material. The contacting surface 4 of the contacthas been treated, as by etching, to remove the lower melting pointconstituent so that the surface 4 comprises the refractory constituentsubstantially free of lowermelting point constituent.

While specific embodiments of the invention have been described, it isintended to cover the invention broadly within the spirit and scope ofthe appended claims. y i

The method of making electric contacts described herein is described andclaimed in our vcopending application Serial No. 533,952, filed positionof tungsten carbide and silver, the contact face thereof beingsubstantially pure tungsten carbide free of silver and having the samemicrostructure as exists throughout the rest of the contact.

3. An electric contact formed of a metal composition of nickel andsilver, the contact face thereof being substantially' pure nickel freeof silver and having the same microstructure` as exists throughout therest of the contact.

4. An electric contact formed of a composition of cadmium oxide andsilver, the contact face thereof being substantially pure cadmium oxidefree of silver and having the same microstructure as exists throughoutthe rest of the contact.

EARL I. LARSEN. VERNON E. HEIL. EARL F. SWAZY.

